Preheat oven to 325 degrees F (165 degrees C). Spray a baking sheet with cooking spray.,Stir buckwheat flour, white sugar, brown rice flour, potato flour, lavender, vanilla powder, and baking powder together in a large bowl, making a well in the center. Whisk eggs, canola oil, and egg white together in a small bowl until well blended. Pour egg mixture into well and stir until dough is combined.,Divide dough in half; form 2 logs with hands. Transfer logs to the prepared baking sheet.,______,Bake in the preheated oven for 10 minutes. Turn cookies over and continue baking until hardened, about 10 minutes more. Remove from oven; cool.